Chemical Engineering Journal 158 (2010) 69–77 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Electrical resistance tomography for characterisation of physical stability in liquid compositions Adam Kowalski a , John Davidson b , Mark Flanagan a , Trevor York b,∗ a Unilever R&D Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral CH63 3JW, UK b University of Manchester, School of Electrical & Electronic Engineering, UK article info Article history: Received 22 April 2008 Received in revised form 19 October 2009 Accepted 23 October 2009 Keywords: Electrical resistance tomography Product ageing 3D image reconstruction abstract The paper describes experiments to investigate the feasibility of using electrical tomography for early characterisation of physical stability in selected products. The Manchester LCT tomograph has been applied to an 8-plane sensor hosted in a 1-l vessel. Measurements have been taken over periods up to 76 h. Tomographic measurements and reconstructed images are consistent with visual observations associated with experiments that readily generate a distinct visible ‘separated layer’. Observed diur- nal excursions in the measured voltages have been investigated and related to changes in temperature. Later experiments have been performed in a temperature controlled environment. Conductivity changes have been extracted from the reconstructed images for selected pixels. These reveal behaviour which is indicative of instability prior to visually discernible effects. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The paper describes work to explore the feasibility of using electrical resistance tomography (ERT) for characterisation of phys- ical stability in multiphase liquid compositions. Fluid preparations which consist of multiple, non-miscible ingredients have a ten- dency to change over time and can suffer from both physical (e.g. creaming) and colloidal (e.g. coalescence) stability problems [1]. Such preparations are common in many formulated products sold to the public including cosmetics, paints, pharmaceuticals, detergents and processed foods. Similarly in business to busi- ness transactions raw material suppliers are increasingly providing their customers with preparations (i.e. suspensions and emulsions) which are easy to handle, incorporate and disperse. For all of these applications the formulated products must remain physically and colloidally stable over timescales ranging from minutes to hours, days, weeks, months and, in the case of some of the products men- tioned above, for several years [2]. Typically, during product development, manufacturers must conduct extensive storage trials over representative timescales in order to assess the impact of formulation and process changes. Such testing is time consuming and labour intensive and often such measurements are not technically demanding (e.g. height of ∗ Corresponding author at: School of Electrical & Electronic Engineering, Univer- sity of Manchester, Sackville Street, PO Box 88, Manchester M60 1QD, UK. Tel.: +44 0161 306 4729; fax: +44 0161 306 4789. E-mail address: t.a.york@manchester.ac.uk (T. York). sediment). Consequently there is a real need for techniques which are capable of either measuring creaming or sedimentation accu- rately and automatically or are able to detect changes which signal the onset of instability. The value of such techniques is in the freeing up of valuable resources (especially research staff) to con- centrate on developing scientific insights which can generate real performance improvements in products or speed up the innovation process. A wide range of measurement techniques have been con- sidered which can be broadly characterised as accelerated testing, point measurement and tomography. Accelerated testing [1] has historically been predominantly used in industry although mod- ern measurement capabilities offer the intriguing possibility of a “high throughput” and/or characterisation of early stage features which are indicative of long term stability. Accelerated testing relies on modifying some external influence, for example grav- itational force, through centrifugation or temperature to reduce viscosity and accelerate colloidal processes. However in practice these techniques can be severely limited in that other physical or colloidal changes may alter stability and give misleading results. For example, the emulsifying power of a non-ionic surfactant is temperature dependent and performance deteriorates or improves with temperature [3,4]. A variety of point or local measurement techniques have also been considered including laser light scatter- ing [5], vibrating reeds [6] and ultrasonics [7]. In many cases the interface (sometimes called the mud line) in a sedimenting sys- tem is diffuse and not easily determined [8]. Point measurement techniques can be positioned at various positions (e.g. [6]) but are only sensitive to local changes whereas tomography offers full 3D monitoring. 1385-8947/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2009.10.054